1. Field of the Invention
This invention relates to a method for manufacturing a luminous tube for discharge lamp, and in particular to a method for sealing components of the luminous tube to each other.
2. Description of the Prior Art
A high pressure discharge lamp, such as metal halide lamp, includes a luminous tube composed of a usually tubular discharge vessel formed of a light-transmissive material, such as quartz glass, and metal halide, mercury, and rare gases as luminous elements sealed therein. At both ends of the discharge vessel are disposed main electrodes made of e.g. tungsten, respectively.
In manufacturing such luminous tube exhaust pipes have conventionally been used. That is, a fine exhaust pipe is provided to the side wall of the quartz glass discharge vessel with the prescribed main electrodes disposed at both ends thereof, and the gas inside the discharge vessel is sucked from the exhaust pipe to keep the internal pressure at 10.sup.-6 torr or below. Thereafter, a rare gas, such as argon is introduced from the exhaust pipe into the discharge vessel to a prescribed pressure, e.g., 20 torr, the predetermined luminous elements are enclosed through a fine pipe, and the exhaust pipe is chipped-off usually by using a gas flame, such as oxygen-hydrogen flame, thus manufacturing the luminous tube.
Thus, in the method using the exhaust pipe, exhaust requires much time and the undesired gases in the vessel may not be fully removed, which may damage the properties of the resultant luminous tube, such as discharge starting voltage. Further, the gas, especially hydrogen, used as the flame source for chippinhg the exhaust pipe may frequently penetrate through the wall of the discharge vessel to be mixed in the vessel or occluded in the vessel wall. The hydrogen mixed in the discharge vessel may adversely affect the discharging condition due to the reaction with the luminous elements in the luminous tube or the action of the hydrogen itself. As for the hydrogen occluded in the vessel wall, it is discharged into the vessel when the luminous tube is operated, causing problems of the same kind. In addition, traces of the exhaust pipe, i.e., projected chipped-off portions will be left, so that an alumina coating layer, as a heat insulator, is applied to such portion to improve the properties of the luminous tube, through such coating layer will reduce the light-transmissivity of the vessel. Moreover, the chipped-off portion may often be thinned, which will lead to explosion of the luminous tube while in use.